This invention relates to an internally threaded fiber-reinforced plastic member and a method of producing the same. More particularly, the present invention is concerned with an internally threaded fiber-reinforced plastic member comprising a hollow body and an integrally formed thread on the inner surface of the bore, and having a unique interior structure in which at least one crossing pair of unified resin-impregnated strand groups is arranged substantially in perpendicular relation to the axis of the member, and also is concerned with a method of producing the same.
Metals or metal alloys such as iron and stainless steel are widely employed as a material for forming threaded members such as bolts and nuts. Metal bolts and nuts are employed as a fastening means in a wide variety of applications including manufacture of playthings and building of large-scale industrial or commercial constructions. However, the use of metal bolts and nuts inevitably encounters serious problems. One of such problems is corrosion. At present, metal bolts and nuts are still often used in corrosive environments. The use of metal bolts and nuts in chemical plants, water or sewage treating plants and the like often leads to fatal dangers. Further, it is noted that metal bolts and nuts are unsuitable for use in marine constructions such as a boat, and artificial reef construction, and the like, since they are caused to incessantly come in contact with the sea water, leading to destruction of the constructions. Another important problem resides in electrical and thermal conductivity of the metal bolts and nuts. There are a number of applications where bolts and nuts are required not to be thermally and electrically conductive. Representative examples of such applications are manufacture of heavy electrical devices or equipments, and construction of electrically-powered transportation devices such as cars for an electrically-powered train, streetcar, etc. For such applications, metal bolts and nuts must be coated with an insulating material. However, the currently employed insulated metal bolts and nuts are still unsatisfactory in various points.
With respect to corrosion resistant metals which can be used as a material for bolts and nuts, it is well known that titanium is a metal which is comparatively resistant to corrosion. Further, due to its small specific gravity, titanium is considered to be useful for forming bolts and nuts. However, titanium is not only expensive but also poor in processability. For this reason, the use of titanium-made bolts and nuts are restricted to special fields, for example, aero-space industry. With respect to electrically non-conductive bolts and nuts, there have recently been proposed bolts and nuts made of ceramics due to the rapid progress of ceramics industry. However, satisfactory ceramic-made bolts and nuts have not yet been developed.
In order to cope with the problem of corrosion, bolts and nuts made of a synthetic resin have been proposed. Such resin-made bolts and nuts, however, are very poor in mechanical strength as compared with those made of a metal, and, hence, cannot be an effective substitute for metal bolts and nuts in the application field where a high mechanical strength is required. Therefore, the application of the resin-made bolts and nuts is restricted to a field where the corrosion resistance is strongly required but the mechanical strength is not a matter of importance.
In the field of manufacture of heavy electrical devices or equipments, as a nut which is required to have excellent mechanical strength and insulating properties, nuts made of fiber-reinforced plastic have been proposed.
With respect to fiber-reinforced plastic nuts conventionally employed, they are generally produced by the following method: A yarn cloth of glass fibers, which has a thickness of 0.1 to 0.25 mm, is impregnated with a resin. The resin-impregnated yarn cloth is then squeezed by passing between rolls, so that the glass fiber content is adjusted to 60% by weight or so. Then, the resin-impregnated cloth is treated to render the resin half-cured. As a result, there is obtained a semi-rigid, substantially non-tacky material (prepreg). A plurality of several tens of such prepregs are piled up in a cavity of a mold, and then subjected to press-molding while heating, whereby a board-like shaped article is obtained. The thus obtained board-like article is cut into a piece having a predetermined shape and a predetermined dimension, followed by boring and threading to obtain a nut.
With the above-mentioned method, in which a yarn cloth having a thickness of 0.1 to 0.25 mm is employed, there cannot be obtained a nut having a sufficient mechanical strength, unless a thread is provided with a pitch of 1 to 2 mm. In the case of a nut of which the thread is provided with a pitch of 1 to 2 mm, the yarn clothes each having a thickness of 0.1 to 0.25 mm can extend into each of the thread and, hence, the reinforcement of the thread can be attained. A nut provided with a thread having a pitch of 1 to 2 mm is suitable for coupling with a bolt having a size of M10 to M25 (the size according to the Japanese Industrial Standards B0205), or larger, of which the thread is formed with a pitch of 1 to 2 mm. That is, with the above-mentioned method, a nut having a sufficient strength is obtained only when the production of a nut adapted to receive the thread portion of a bolt having a size of M10 to M25, or larger is intended. Further, this method has the following disadvantages. It is troublesome and takes a lot of time to pile up in a cavity of a mold a large number (generally, 30 to 80) of prepregs prior to pressing. In addition, it is required to use an expensive yarn cloth. Hence, according to the abovementioned conventional method, it is impossible to provide a nut which exhibits a high mechanical strength and can be produced with high productivity and at low cost.